This invention pertains to scannable wireless receivers generally. Without limitation, the background is described in connection with FM receivers. FM is popular in many developed countries and is growing in popularity in a number of developing countries.
In the United States and Europe, FM broadcast stations use a bandwidth of 200 KHz assigned to them at different frequencies or positions within the 87.5 MHz to 108 MHz. In Japan the FM band or available frequency spectrum is a 76 MHz to 90 MHz band. There, an FM channel can be centered at multiples of 50 KHz, with a frequency spacing of at least 200 KHz between any two valid stations. The FM center frequency can be centered at multiples of 50 KHz in some parts of the world and at multiples of 100 KHz in other parts of the world. Hence, scanning for FM bands at multiples of 50 KHz or multiples of 100 KHz are two useful operations.
With the growing popularity of FM transmission all over the world, low-cost integrated FM receivers have become important to integrate into mobile handsets like cell phones and Internet devices as well as FM-supporting integrated circuits of various types for those and other products.
In an FM receiver, band scan time herein is defined as the total time taken to scan the entire FM band, in the absence of any valid FM broadcast stations. For example, a tune time of 40 ms/channel over 200 stations can result in a total band scan time of 8 seconds, which may constrain or limit the user experience for at least some users.
The time taken to scan the entire band and automatically identify valid FM broadcast station is a key care-about as it is perceivable by the user. To cover the entire FM band from 87.5 MHz to 108 MHz calls for time-intensive scanning as many as 200 station slots at 100 KHz spacing. In some countries, the FM center frequencies can occupy a 50 KHz raster, calling for autonomous scan with a frequency spacing of 50 KHz, which doubles the FM band scan time, all other things being equal. Thus, the FM band scan time is problematically longer there, in the sense of a numerical bound, by the time taken to tune to each of the about 400 channels (at 50 KHz channel spacing) and identify the presence of a valid station.
Hitherto, it is believed that an approach involved a frequency synthesizer to scan channel-by-channel for an FM station and lock on to it, and that such approach attempted to reduce synthesizer lock time by using a higher reference clock rate. Unfortunately, such approach is not believed to accommodate a low clock rate 32 KHz reference clock. Low power mobile handset applications and other microprocessor-supported applications commonly use a 32 KHz reference clock such as for a real-time clock (RTC), and finding some distinct departures to rapidly scan FM stations in a low power mobile handset or other microprocessor-supported applications would be most desirable.